Sweden's most out-of-the-way airport, Kiruna in the Arctic Circle, is an ideal proving ground for ADS surveillance systems

Passengers arriving at Kiruna airport in Sweden's Arctic region are treated to a 15min sightseeing flight over the region's top attractions. Yet Scandinavian Airlines (SAS), which runs the twice-daily service from Stockholm Arlanda, is not performing the elaborate figure-of-eight routine, taking in the Kebnekaise mountain and Jukkasjärvi ice hotel, to aid local tourism: it is because the non-radar airport requires a full procedural approach.

Even a country as prosperous as Sweden cannot afford to extend radar coverage to Kiruna, a small mining town lying at 67° 49°N. Niclas Gustavsson, air traffic management (ATM) planning and development manager for the air navigation services division of the Swedish LFV civil aviation administration, says the country first pursued research into alternatives to radar in the early 1990s due to budgetary issues. "Now that the aviation sector is suffering, there is a pressure on air navigation service providers to reduce costs, and the use of new technologies is one way to achieve this," he says.

Visibility at the airport is often poor in Kiruna, with little daylight in winter and frequent bad weather, so visual approaches are rare. Additionally, attracted by Kiruna's winter temperatures of –40°C (-40°F), the town plays host to regular cold-weather equipment testing, with teams arriving on chartered flights with pilots unfamiliar with the airfield. Thus, procedural turns into a VOR/DME are standard even in clear weather. However, with as few as nine movements a day during the winter, Kiruna is hardly the most congested airport in Sweden, which is why the LFV is using it to test a cheap alternative to radar.

ADS system

The Lapland airport is equipped with a system based on automatic dependent surveillance (ADS) for the radar-like display of aircraft positions within a 370km (200nm) range of the airport.

ADS interfaces with existing avionics and transmits aircraft identification, four-dimensional position, velocity, and other data. Automatic dependent surveillance - broadcast (ADS-B) transmits information that can be received by any suitably equipped air- or ground-based user; while a point-to-point closed-communication version, ADS-C, is being developed using the same protocol. Unlike radar, the ADS ground station is passive, receiving position reports based on aircraft's flight management system.

At Kiruna, which uses procedural air traffic control below FL70 (7,000ft/ 2,120m), the advantages of ADS-B are clear, says airport operations manager Ulf Mukka. Airlines need a better surveillance service, mainly en route, but also in the terminal manoeuvring area (TMA), to provide flexible approaches, he says.

LFV started trials of ADS-B around 15 years ago, but impetus was added in 1997 with the results of an internal report showing that an entire ADS-B network could be installed in Sweden for less than the cost of one replacement secondary surveillance radar, says Gustavsson. As a result, the LFV headed the European Commission-funded four-nation North European ADS-B Network (NEAN) project, looking more closely at the benefits and costs of the system.

NEAN quickly proved the system's potential and in 1999 Sweden led other nations into two phases of NEAN Update Programmes (NUP) designed to define and validate the system. Kiruna, along with sites in Austria and Greenland, was chosen for live trials of ADS-B in a non-radar environment. A Cessna 172 Skyhawk and a Eurocopter EC120 Colibri were both fitted with transponders, and their movements tracked against pilots' verbal position reports. Air traffic controllers comparing the ADS-B display to an en route radar feed from Luleå, 300km to the south, report the ADS-B data to "always be more accurate" than the secondary radar.

In 2002, the airport also played host to six missions by an ADS-B-equipped unmanned air vehicle, which maintained contact with the tower throughout flights lasting 1-5h and reaching an altitude of 20,000ft. "The goal was for the UAV to share the same block of airspace as our general aviation and scheduled traffic," says Owe Odin, ADS-B Kiruna project manager for the LFV.

Other trials under the NUP are looking at a variety of additional applications: ADS-B for rotorcraft operations using North Sea offshore helicopters; airport surface-movement operations; air-to-air applications; and air traffic control integration.

Scandinavia is suited to the use of ADS-B as a replacement for secondary radar and, as such, the LFV is conducting several other trials. Several general aviation aircraft operating from the authority's home base of Norrköping are fitted with transponders. Displays allow them to monitor each other's position during flight. Meanwhile, another trial at Stockholm Arlanda has 17 ground vehicles equipped with ADS-B transponders, with the data to be compared with a millimetre-wave surface movement radar (MMR) used to monitor runway incursions. At the moment only those vehicles routinely used on the runways, such as follow-me cars, friction testers and fire trucks, are equipped with transponders, but eventually all vehicles in the airport will have them.

Gustavsson says: "On the ground, ADS-B provides a much simpler surveillance infrastructure compared with traditional ground radar, it is much cheaper and, crucially, has no shadows." At this time of year, for example, the MMR system at Arlanda suffers from black spots behind snow drifts. Gustavsson estimates that radar covers 95% of the apron's surface area and ADS-B 100%. Aircraft intent and status information is also available on the ground via ADS-B, which should make ATM more precise, he adds.

On the buses

In a commercial spin-off, the airport's passenger bus fleet has been equipped with transponders and the operator is using the system as a sophisticated dispatcher. Joakim Käll, the ATM research and development engineer tasked with overseeing the bus project, says: "Passenger buses need access to exactly the same information as controllers, such as estimated arrival time, stand number and passenger numbers; so the bus fleet operator asked us to fit ADS-B terminals to improve their efficiency and reduce time waiting for aircraft."

ADS-B should be a key component in collaborative decision-making at airports, says Gustavsson. The LFV estimates that airport delays could be reduced by around one-third if pilots, air traffic controllers and ground service providers all had access to common data. "Accurate landing times, gate change details and shared de-icing information will all reduce delays," says Käll.

In its first phase, ADS data broadcasts will be one-way, from the transponder to the ground station. In phase two - "at least five years away", according to Gustavsson - two-way broadcasts will allow airborne separation assurance system (ASAS) advice and partial delegation of separation authority to pilots. By phase three, the LFV envisages full automatic ASAS as well as collision detection and avoidance.

For this to happen, there has to be global standardisation on a datalink format. ADS-B can be used with any format, but the LFV has chosen to couple the system with VHF datalink Mode 4 (VDL-4).

International Civil Aviation Organisation standards stipulate both VDL-4 and Mode S as accepted ADS-B datalinks, and look likely to add a third in the form of the Universal Access Transceiver (UAT). The three datalinks are not interoperable and there is wide debate on the merits of each.

Sweden is clearly in favour of VDL-4, with the equipping of regional carrier Malm" Aviation's nine BAe Avro RJ100s next month likely to be followed by the installation of ADS-B avionics on SAS aircraft. In addition to Arlanda, Copenhagen and Oslo airports will acquire ADS-B ground stations as part of a gate-to-gate pre-implementation study involving busy air corridors.

Modes preferred

However, since ADS-B is dependent on receiving position data either from an aircraft's flight management system or global positioning systems, secondary radar is viewed as less risky for separation and as such, Mode S is likely to remain in place in Europe's denser airspace. Additionally, as all passenger aircraft with more than 30 seats are required to fit traffic collision avoidance systems (TCAS) that include Mode S extended squitter, it is likely this will become the standard datalink for large aircraft. But since VDL-4 systems can simply replace low-power VHF radio, however, general aviation aircraft in Europe are likely to adopt the format, with UAT used for smaller types in North America.

ADS-B also lacks standardisation, with several proprietary display, software and network protocols being tested around the world. However, with ICAO now backing ADS-B, manufacturers are finally getting behind the system, says Gustavsson.

The LFV's safety division is likely to certificate the first ADS-B equipment this year and the first operational use of the system will probably be at Kiruna, where scheduled carriers are being equipped. Once the system is approved for use next year, controllers will approve precision approaches using vectors to the airport, saving aircraft up to 10min of fuel. The stunning views, however, will be missed.

Source: Flight International